The present invention relates to an apparatus for applying tension to a tubular member and to a method of using the same. In particular, the present invention relates to an apparatus for tensioning tubular members used in offshore oil and gas drilling and production operations, for example the legs of tensions leg platforms, casing strings and risers, and a method for carrying out the same.
Many situations require a tubular member to be placed under tension. Such situations arise in many different aspects of the operations for exploration, drilling and production of oil and gas, in particular in offshore locations. Examples of situations requiring tubular members to be tensioned include the setting of tension leg platforms for offshore drilling and production operations. Further examples arise in the drilling and production of oil and gas from wells accessed through wellheads located on the sea floor, and include the tensioning of risers and casing strings extending between a hanger located in a wellhead on the seabed and a wellhead mounted on a platform or vessel at the surface.
U.S. Pat. No. 4,794,988 discloses a surface wellhead apparatus for use in tying back casings extending to a subsea structure. The casing is held under tension by a lock member which engages on a shoulder within the surface wellhead. A similar arrangement is described in U.S. Pat. No. 4,938,289. Both arrangements require the casing string to be first placed under tension, after which the casing may be held in tension using the arrangements disclosed.
U.S. Pat. No. 4,995,464 describes an offshore well installation in which an adjustable assembly is employed to tension a casing string or other tubular member. The installation comprises a first tubular member and a second tubular member arranged concentrically with a sleeve disposed therebetween. The sleeve is formed with an inner and outer thread thereon, engaging with corresponding outer and inner threads on the first and second tubular members. A lug is disposed between the first and second tubular members to prevent relative rotation of one against the other. Rotation of the sleeve moves the first and second sleeves longitudinally with respect to one another, thus allowing a tension to be placed on the casing string. While the arrangement can be operated to apply tension to the casing string without requiring the string to be tensioned by other means, this is only possible by rotation of the sleeve.
U.S. Pat. No. 5,638,903 discloses an adjustable mandrel hanger system for maintaining tension in a string of casing extending between a subsea wellhead assembly and a surface wellhead housing. A mandrel is secured to the end of the casing string, to which is mounted a locking member. The locking member lands against a shoulder in the wellhead housing. Upon installation, the operator applies tension to the casing string. The mandrel moves upwards relative to the locking member as the tension is applied. Upon release of the tension applied by the operator, the locking member will retain the mandrel and the string under tension against the shoulder. It is to be noted that the hanger system of U.S. Pat. No. 5,638,903 simply holds a casing string under tension, once the tension has been applied by the operator. The hanger system cannot itself be used to apply tension to the casing string.
U.S. Pat. No. 5,653,289 discloses a casing tensioning system for applying tension to a string of casing between a subsea wellhead and a surface wellhead. A casing hanger is secured to the casing: string. The casing hanger has a first position, allowing downward movement of the casing string with respect to the hanger, and a second position, in which upward movement of the casing string is allowed, but in which downward movement of the string relative to the hanger is prevented. The hanger is landed on a shoulder within the surface wellhead. The operator applies tension to the casing string, after which the casing hanger acts to retain the casing string under tension. Again, while the apparatus disclosed is sufficient to hold the tensioned casing string, it cannot be operated to apply the required tension to the string or another tubular member.
A similar arrangement is described in U.S. Pat. No. 5,671,812, in which a casing hanger is secured to a mandrel, the mandrel in turn being attached to a casing string to be tensioned. Again, the casing hanger allows upwards movement of the mandrel with respect to the hanger, but prevents relative downward movement of the mandrel. Hydraulic pressure is used to force the casing hanger to seat against a shoulder within the surface wellhead, after which the mandrel is raised, thereby placing the casing string under tension. As with the earlier designs discussed above, the casing hanger, while retaining the casing string under tension once sufficient tension has been applied, does not act itself to apply tension to the casing string.
An arrangement similar to that of U.S. Pat. No. 5,671,812 is disclosed in U.S. Pat. No. 5,944,111, with the difference that a launch adaptor is used to force the casing hanger against the shoulder in the surface wellhead, after which tension is applied to the casing. The casing hanger acts to retain the string of casing under tension in a similar manner to that described in U.S. Pat. No. 5,671,812.
It can be seen that a variety of assemblies have been proposed to retain a tubular member, such as a casing string, under tension between two fixed assemblies. However, in such arrangements, it is necessary to provide additional means to place the tubular member under the required tension. U.S. Pat. No. 4,995,464 discloses an arrangement in which a single assembly is employed to both apply tension to a tubular member, in this instance a casing string, and retain the tubular member under tension, once applied. However, this arrangement only operates by the interaction of a plurality of separate threads formed on various of the tubular components. The machining of threads is time consuming and undesirable.
Accordingly, it can be seen that there is a need for an assembly which can be attached to a tubular member, such as a string of casing in an offshore well, and operated to both apply tension to the tubular member and retain the member under tension, once applied, by linear movement of the components of the assembly and without the need for components to be rotated or formed with threads.
According to a first aspect of the present invention, there is provided an apparatus for securing a tubular member under tension, the member secured to and extending between a first and second, fixed assembly, the apparatus comprising:
a first tubular assembly connectable at a first engageable portion to the tubular member to be tensioned;
a second tubular assembly having a first and a second engageable portion, the second tubular assembly being arranged concentrically with respect to the first tubular assembly and movable longitudinally with respect to the first tubular assembly;
a first engagement assembly for engaging the first engageable portion of the second tubular assembly with the first fixed assembly, such that movement of the second tubular assembly toward the tubular member to be tensioned is prevented;
a second engagement assembly for engaging the second engageable portion of the second tubular assembly with the first tubular assembly upon longitudinal movement of the second engageable portion of the second tubular assembly with respect to the first tubular assembly toward the first engageable portion of the first tubular assembly;
wherein the second tubular assembly can be placed under tension by moving its second engageable portion longitudinally relative to its first engageable portion, when the first engageable portion is engaged with the first fixed assembly.
When the first tubular assembly is attached to a tubular member, such as a casing string or riser, longitudinal movement of the second engageable portion of the second tubular assembly toward the tubular member applies tension to the second tubular assembly, in turn tensioning the tubular member. The apparatus of the present invention may thus be installed to secure an end of the tubular member, for example a casing string or riser, to a first fixed assembly, for example a surface wellhead. Once the tubular member has been attached to the second fixed assembly, for example a subsea wellhead, the apparatus may also be used to tension the tubular member, without the need for additional tensioning equipment. The tension is applied by moving the second engageable portion of the second tubular assembly in a longitudinal direction and, thus, does not require any of the components to be rotated or be formed with any additional threaded sections, other than those conventionally found in such systems. This is turn allows the apparatus to be manufactured in a simple manner.
In a preferred embodiment, the second engageable portion of the second tubular assembly extends concentrically within the first tubular assembly.
The first engagement assembly comprises a locking collar, the locking collar for engaging a shoulder in the first fixed assembly. In this arrangement, the locking collar simply bears against the shoulder, in order to prevent the second tubular assembly from moving towards the tubular member to be secured and tensioned. The first engagement assembly may further comprise a locking ring, for engaging a groove in the first fixed assembly. The locking ring may be biased into engagement with the groove. In an alternative arrangement, the locking collar is movable longitudinally with respect to the second tubular assembly upon contact with the shoulder in the first fixed assembly, such movement urging the locking ring into engagement with the groove in the first fixed assembly.
Preferably, the second engagement assembly allows the second engageable portion of the second tubular assembly, when engaged with the first tubular assembly, to move away from the first engageable portion of the second tubular assembly.
In a preferred embodiment, the second engagement assembly allows the second engageable portion of the second tubular assembly to engage with the first tubular assembly in one of a plurality different positions. This arrangement allows the tension being applied to the tubular member to be varied, while still allowing the first and second tubular assemblies to engage, in turn securing the tubular member to the first fixed assembly.
Preferably, the second engagement assembly has a first operating mode, in which engagement between the first and second tubular assemblies is not possible, and a second operating mode, in which engagement between the first and second tubular assemblies is possible. In this way, the second engagement assembly may be held inoperative, until the necessary steps have been taken to secure the apparatus to the tubular member to be secured and tensioned and until the second tubular assembly has been engaged with the first fixed assembly by the first engagement assembly. Most preferably, the second engagement assembly is moved from the first operating mode to the second operating mode upon the application of a predetermined tension to the second tubular assembly.
In a specific embodiment of the apparatus of the present invention the second engagement assembly comprises a first groove in the first tubular assembly and a second groove in the second tubular assembly, the second engagement assembly further comprising a locking ring for engaging both the first groove and the second groove. Preferably, a plurality of first grooves are provided, thereby allowing the second engageable portion of the second tubular assembly to engage the first tubular assembly in a plurality of different positions. The locking ring of the second engagement assembly may be held completely within the second groove in the second tubular assembly until a predetermined tension is applied to the second tubular assembly.
A tensioning collar may be provided in the second tubular assembly at its second engageable portion, the tensioning collar being engageable by a tool for tensioning the second tubular assembly. If present, the tensioning collar is preferably movable between a first position, in which the tensioning collar holds the locking ring completely within the second groove, and a second position, in which the locking ring is released to engage the first groove. The locking ring is preferably biased into engagement with the first groove, the tensioning collar holding the locking ring against its bias in the first position. The tensioning collar may be arranged to move from the first position into the second position at a predetermined tension applied to the second tubular assembly.
The apparatus of the present invention may be used to secure and tension tubular members in general. However, the apparatus finds particularly advantageous application in the securing and tensioning of casing string, risers, and the legs of a tension leg platforms.
In a further aspect, the present invention provides a method for securing and tensioning a tubular member, the tubular member extending between a first fixed assembly and a second fixed assembly, the method comprising:
securing a first tubular assembly to the tubular member;
providing a second tubular assembly, having a first engageable portion and a second engageable portion;
securing the second tubular assembly at its first engageable portion to the first fixed assembly, such that the first tubular assembly is prevented from moving towards the tubular member;
applying tension to the second tubular assembly by moving the second engageable portion away from the first engageable portion by applying a force longitudinally to the second tubular assembly;
engaging the second engageable portion of the second tubular assembly with the first tubular assembly.
The second tubular assembly is preferably moved longitudinally from a disengaged position to an engaged position, in which the second tubular assembly is engaged with the first fixed assembly, the longitudinal movement of the second tubular assembly being continued to tension the second tubular assembly and engage the second tubular assembly with the first tubular assembly.
It is advantageous if the engagement of the second engageable portion of the second tubular assembly is carried out selectively, when the second tubular assembly has been position appropriately with respect to the first tubular assembly, prior to which the engagement of the two assemblies not being possible. In a preferred embodiment, the second engageable portion of the second tubular assembly is engaged with the first tubular assembly upon application of a predetermined tension to the second tubular assembly.
Preferably, an engagement assembly is provided to engage the second engageable portion of the second tubular assembly with the first tubular assembly, the engagement assembly being biased into an engaged position, the engagement assembly being held in a disengaged position until application of the predetermined tension to the second tubular assembly.
In a preferred embodiment, the second engageable portion of the second tubular assembly is engageable with first tubular member in a plurality of positions. In this way, the tension applied to the tubular member may be varied, as required by the prevailing circumstances.
In a further aspect, the present invention provides an apparatus for use as a tool for securing and tensioning a tubular member, such as a casing string or riser. Accordingly, an apparatus for securing and tensioning a tubular member in a first fixed assembly, the apparatus comprising:
a first engagement assembly, for securing the apparatus with respect to the tubular member;
a second engagement assembly for engaging a tubular assembly and applying tension to the tubular assembly by longitudinal movement towards the tubular member.
In the apparatus, longitudinal movement of the second engagement assembly preferably secures the tubular assembly in the first fixed assembly, after which continued longitudinal movement applies tension to the tubular assembly.
In a preferred embodiment, the apparatus further comprises a piston, the second engagement assembly being attached to the piston, the piston being moveable in a longitudinal direction with respect to the tubular assembly. The piston is most conveniently moved by means of a hydraulic fluid.
Specific embodiments of the apparatus and method of the present invention will now be described in detail having reference to the accompanying drawings. The detailed description of these embodiments and the referenced drawings are by way of example only and are not intended to limit the scope of the present invention.